Add decimalPatternDigits to gen-l10n valid NumberFormats (#122682)

`intl` [0.18.0 added](https://pub.dev/packages/intl/changelog#0180) a
[`NumberFormat.decimalPatternDigits`](https://pub.dev/documentation/intl/latest/intl/NumberFormat/NumberFormat.decimalPatternDigits.html)
pattern that allows `decimalDigits` param.

Fixes #122785

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [ ] ~I updated/added relevant documentation (doc comments with
`///`).~
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[test-exempt]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
[Discord]: https://github.com/flutter/flutter/wiki/Chat

packages/flutter_tools/lib/src/localizations/gen_l10n_types.dart

@@ -94,6 +94,7 @@ const Set<String> _validNumberFormats = <String>{
   'compactLong',
   'currency',
   'decimalPattern',
+  'decimalPatternDigits',
   'decimalPercentPattern',
   'percentPattern',
   'scientificPattern',
@@ -118,6 +119,7 @@ const Set<String> _numberFormatsWithNamedParameters = <String>{
   'compactSimpleCurrency',
   'compactLong',
   'currency',
+  'decimalPatternDigits',
   'decimalPercentPattern',
   'simpleCurrency',
 };

packages/flutter_tools/test/general.shard/generate_localizations_test.dart

@@ -3196,4 +3196,52 @@ AppLocalizations lookupAppLocalizations(Locale locale) {
       ..writeOutputFiles();
     expect(logger.hadWarningOutput, isFalse);
   });
+
+  testWithoutContext('can use decimalPatternDigits with decimalDigits optional parameter', () {
+    const String arbFile = '''
+{
+  "treeHeight": "Tree height is {height}m.",
+  "@treeHeight": {
+    "placeholders": {
+      "height": {
+        "type": "double",
+        "format": "decimalPatternDigits",
+        "optionalParameters": {
+          "decimalDigits": 3
+        }
+      }
+    }
+  }
+}''';
+
+    final Directory l10nDirectory = fs.currentDirectory.childDirectory('lib').childDirectory('l10n')
+      ..createSync(recursive: true);
+    l10nDirectory.childFile(defaultTemplateArbFileName)
+        .writeAsStringSync(arbFile);
+
+    LocalizationsGenerator(
+      fileSystem: fs,
+      inputPathString: defaultL10nPathString,
+      outputPathString: defaultL10nPathString,
+      templateArbFileName: defaultTemplateArbFileName,
+      outputFileString: defaultOutputFileString,
+      classNameString: defaultClassNameString,
+      logger: logger,
+    )
+      ..loadResources()
+      ..writeOutputFiles();
+
+    final String localizationsFile = fs.file(
+      fs.path.join(syntheticL10nPackagePath, 'output-localization-file_en.dart'),
+    ).readAsStringSync();
+    expect(localizationsFile, containsIgnoringWhitespace(r'''
+String treeHeight(double height) {
+'''));
+    expect(localizationsFile, containsIgnoringWhitespace(r'''
+NumberFormat.decimalPatternDigits(
+  locale: localeName,
+  decimalDigits: 3
+);
+'''));
+  });
 }